Methods of lining pipes have already been proposed wherein a rigid plastic tube inserted through the pipe is inflated radially of the pipe by applying heat and pressure to the tube from inside to thereby line the pipe with the tube (see, for example, Unexamined Japanese Patent Publication No. 88281/1983).
The rigid tube lining formed by the above method is hard, therefore retains itself in the pipe lining state and is better in quality than the soft tube lining which is bonded with an adhesive to the pipe for lining.
With the rigid tube lining method, the rigid plastic tube inserted through the pipe is softened by heating before the pressure inflation step so as to be inflatable by application of pressure.
To soften the rigid plastic tube by heating, it is proposed to use a heating fluid such as steam to the inside of the tube. The heating fluid serving as the softening means is advantageous over a heating jig having an electric heater incorporated therein in that the tube can be heated and softened more easily in respect of the device and procedure needed.
The heating fluid serving as for heating-softening means poses no particular problem insofar as the rigid plastic tube to be treated has a relatively small wall thickness. However, problems arise when this means is used for heating and softening tubes having a relatively large wall thickness, e.g. about 5 to about 20 mm, for use in lining pipes, such as water supply mains, having a relatively large diameter, e.g. about 200 to about 800 mm. When the inner peripheral portion of the tube heated by direct contact with the heating fluid is compared with the outer peripheral portion thereof which is not in direct contact with the fluid, the outer peripheral portion is softened noticeably less effectively than the inner peripheral portion. This is attributable to low heat conductivity of the plastic and also to the release of heat from the outer periphery. The difference in the softened state between the inner peripheral portion and the outer peripheral portion leads to insufficient inflation, rupture or similar problems when the tube is inflated by application of pressure.
To compensate for the insufficient heating of the tube outer peripheral portion in this case, it appears useful to heat the tube from outside in addition to heating from inside, but this is not very effective due to marked dissipation of heat from the tube toward the pipe to be lined.